Fast Facts

Female ammonites grew up to 400 percent larger than males, presumably to make room to lay eggs.

Size relative to a 6-ft (2-m) man:

Ammonites were predatory, squidlike creatures that lived inside coil-shaped shells. Like other cephalopods, ammonites had sharp, beaklike jaws inside a ring of tentacles that extended from their shells to snare prey such as small fish and crustaceans. Some ammonites grew more than three feet (one meter) across—possible snack food for the giant mosasaur Tylosaurus.

Ammonites constantly built new shell as they grew, but only lived in the outer chamber. They scooted through the warm, shallow seas by squirting jets of water from their bodies. A thin, tubelike structure called a siphuncle reached into the interior chambers to pump and siphon air and helped them move through the water.

Ammonites first appeared about 240 million years ago, though they descended from straight-shelled cephalopods called bacrites that date back to the Devonian, about 415 million years ago. Ammonites were prolific breeders, lived in schools, and are among the most abundant fossils found today. They went extinct with the dinosaurs 65 million years ago. Scientists use the various shapes and sizes of ammonite shells that appeared and disappeared through the ages to date other fossils.